(CNN) -- After first saying that they could not determine a threshold for the safe amount of certain toxic chemicals in infant formula, Food and Drug Administration officials said Friday that trace amounts are safe.

Worries over melamine in infant formula started in China and have spread to the United States.

"Amounts of the industrial chemical melamine or the melamine-like compound called cyanuric acid that are below 1.0 ppm [1,000 parts per billion] do not raise public health concerns," said Stephen Sundlof, the FDA's director of the Center for Food Safety and Applied Nutrition.

When it became known in September that thousands of babies in China had been sickened by tainted infant formula, the FDA ordered the testing of U.S.-manufactured infant-formula products.

Sundlof said Friday that results from 74 of 87 samples of infant formula and other products babies and young children had been completed. The results of the other 13 are pending.

That total differed slightly from the one the FDA offered Thursday, when its acting commissioner of public affairs, Judy Leon, told CNN that 77 sample results had been completed and just 10 were outstanding.

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Among the 74 final test results discussed Friday, the FDA said, two samples of Nestle's Good Start Supreme Infant Formula with Iron tested positive for melamine at levels of 137 and 140 parts per billion, well below the level of concern cited by Sundlof.

In addition, three samples of Mead Johnson's infant powder, Enfamil LIPIL with Iron, tested positive for cyanuric acid at an average of 247 parts per billion, also well below the FDA trace level.

The FDA's October 3 risk assessment of melamine and its analogues concluded that "levels of melamine and its analogues below 2.5 ppm in foods other than infant formula do not raise public health concerns."

But the agency said then that it did not have enough data to determine a safe level of melamine and its analogues in infant formula.

On Friday, an FDA official said it was still not possible to determine a safe level for infant formula containing both melamine and cyanuric acid compounds, but officials "believe that at very low levels there should not be any health concerns."

Sundlof said the trace levels of melamine or related compounds detected in the samples are 10,000-fold less than the amount associated with Chinese infant-formula contamination.

According to the World Health Organization, contaminated infant formula in China has killed four babies and caused more than 47,000 infants and children to be hospitalized with possible kidney stones.

Chinese-made infant formula is not available in the United States, because no Chinese manufacturer meets the requirements to sell infant formula in this country, according to the FDA.

It was also the combination of melamine and cyanuric acid in pet food that caused thousands of dogs and cats to be sickened last year in the United States.

The combination caused crystals to form in the animals' kidneys.

According to the FDA's updated Interim Safety Assessment, infants are more vulnerable to the combination than are older children or adults because formula could be the infants' sole source of nutrition. In addition, their kidneys may not have developed sufficiently to ward off the impact of the chemicals.